Wind is a clean, renewable energy source, but there isn't nearly enough of it down here on the ground. But high in the sky, there are furious jet streams of powerful wind just waiting to be harvested by floating turbines.

NASA engineer Mark Moore has been working on the idea of airborne wind turbines for years. He imagines massive turbines as high as 30,000 feet in the air tethered to the ground by long, nearly invisible nanotubes. The turbines would harvest the power of the jet streams and then transmit the power down the nanotubes back towards Earth. He's now got a federal grant to figure out just how feasible these ideas really are.

In theory, the energy benefit of airborne turbines is enormous. The amount of available energy to be extracted from wind increases exponentially the higher up you go, and wind is much more constant at high altitudes than it is near the ground. Moore provides some idea of the tremendous energy benefits available:

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"At 2,000 feet (610 m), there is two to three times the wind velocity compared to ground level. The power goes up with the cube of that wind velocity, so it's eight to 27 times the power production just by getting 2,000 feet (610 m) up, and the wind velocity is more consistent. [At 30,000 feet], instead of 500 watts per meter (for ground-based wind turbines), you're talking about 20,000, 40,000 watts per square meter. That's very high energy density and potentially lower cost wind energy because of the 50-plus fold increase in energy density."

Of course, that wouldn't solve our energy problems, but it would make wind energy a far bigger player in the solution. Also, even if it would cost money to get the turbines up in the air, the actual operating costs would be far cheaper than any other energy source:

"They could stay up a year, then come down for a maintenance check and then go back up. Or they could be reeled in in case of a storm. Or one operator could watch over 100 of these.

There's also the question of land to consider. Most other power plants take up a huge swath of usable land. These turbines, on the other hand, wouldn't take up much more space than it takes to secure the nanotube tether.

Still, there are some drawbacks. One of the major problems is where to put these things. Airplanes won't be able to go near the turbines, and airspace is already crowded enough. Moore explains:

"Airspace is a commodity. You have to be able to use airspace without disrupting it for other players. Smaller aircraft are still going to need to fly around. Larger airplanes, you can't expect them to fly around every wind turbine that has a two-mile radius as a protected flight zone.

Moore thinks he might have a solution:

"Offshore deployment of these airborne systems probably makes the most sense in terms of both airspace and land use, because there is little to no demand for low altitude flight over oceans 12 miles (19 to 20 km) offshore. Also, unlike ground-based turbines, there is almost no additional cost for airborne systems offshore because huge platforms are not required to support the structure or resist large tower bending moments."

We probably won't see any turbines taking to the air for a few more years yet, but you can get some idea of the technology with the concept drawings we presented in a post a few years back. You can see a couple of our favorites above.